Cellular wireless data access systems typically consist of a set of base stations each with a certain number of sectors and each sector having a transmitter and a receiver with one or multiple antenna elements. Mobile terminals are typically assigned to one or more base stations by wireless connections to one sector of each of these base stations. Base stations and sectors are related to geographical areas where the distance to the antennas belonging to the sectors is low, the angular range supported by the antenna corresponds to that area and the channel quality between users located in that area and the base station including interference from other terminals/base station is sufficient to establish a wireless connection.
Connections between a base station and mobile terminals that are assigned to the same sector of that base station can typically have significantly different channel qualities and very different path losses depending on their distance to the base station, which typically varies from a few meters up to a few kilometers. This means that a signal transmitted by the base station at a certain power level can have significantly different power levels at the receivers of different terminals due to different path losses, and signals transmitted by different users in different distances to the base station that arrive at the base station with the same power level must have been transmitted with different power levels at the terminals.
The channel between the base station on the one side and a variety of users in different distances and with different channel properties on the other side is in the following called multi-user channel. In downlink it is a broadcast channel with one input fed by a base station transmitter and many outputs listened to by mobile terminal receivers. In uplink it is a multiple-access channel with many inputs fed by mobile terminal transmitters and one output to the base station receiver. The channel between the base station and different users in different distances can be regarded as a series connection of channel sections with different channel qualities measured e.g. in form of signal-to-noise-and-interference ratios (SINRs) or fading channel characteristics. In such a model the interface between the sections is a spatially distributed signal and the signal seen by a terminal located at said interface is a scalar or vector signal derived from said spatial signal e.g. by antennas.
In current broadband wireless access systems, like for example IEEE 802.16, also known as WiMAX, using OFDM/OFDMA, each single radio resource (frequency sub-band or time slot) is used in one point-to-point transmission. Multiplexing and multiple access is done using different at least approximately orthogonal radio resources. For transmitting broadcast or multicast information, like for example maps required for the allocation of radio resources to terminals and for user data to or from far terminals with high path loss, a robust modulation and coding method with low spectral efficiency is used.
Thus, in known systems a multi-user channel is used in time and frequency multiplex, which is not optimal with respect to throughput in relation to theoretical capacity. In fact, capacity is wasted and the known transmission methods do by far not achieve the information theoretical sum capacity of the multi-user channel in a cellular wireless access system. This results in poor spectral efficiency and low average throughput.
It is an object of the present invention to improve the usage of a multi-user channel and thus to enhance the throughput and spectral efficiency in wireless access systems having a strongly location dependent channel quality.